1. Field of the Invention
The present invention relates to a digital signal decoding apparatus and a method thereof, for use in a digital communication apparatus including a satellite or an optical submarine cable, and a digital recording/reproducing apparatus using a digital VTR, a digital video disc, or the like.
2. Description of the Related Art
In relation to a digital signal decoding apparatus, a Viterbi decoding method is one of maximum likelihood decoding methods which make a full use of information included in a signal. This method is conventionally used as a detection method on which various studies and researches have been made mainly in the filed of digital communication using a satellite, a submarine cable and the like. Meanwhile, in relation to a digital VTR or a digital video disc, it is necessary to increase the recording density since a digital signal used in these digital recording medium must include an extremely large amount of data which is as several times as large as the data amount of an analogue signal. Such a communication apparatus or recording/reproducing apparatus using high technologies, a received or reproduced signal has a very low S/N ratio, and it is therefore very difficult to use a conventional method of decoding signals for every one bit. Hence, it is required to decode a signal by making a full use of data included in the signals of low S/N ratios as much as possible. Particularly, the Viterbi decoding method stated above attracts public attention since the S/N ratio can practically be raised for about 3 dB by using this method.
As a simple example of the Viterbi decoding method, application of this method to an NRZI method will be explained below.
In an NRZI method, an exclusive OR operation value b.sub.k is prepared from an input signal a.sub.k and a delay signal b.sub.k-1, as shown in a pre-code block shown in FIG. 6, and the operation value is recorded into a magnetic recording device. EQU b.sub.k =a.sub.k (+) b.sub.k-1 ( 1)
When this value is reproduced, a reproduction signal z.sub.k is obtained by b.sub.k -b.sub.k-1 since a magnetic recording system has a differential characteristic. The signal system of this reproduce signal have two states, i.e., S.sub.k ={+1, -1} exists. EQU z.sub.k =b.sub.k -b.sub.k-1 ( 2)
This equation can be expressed as FIG. 7 in form of a state shifting view. When the reproduction signal z is +2, the state S shifts from -1 to +1; when the reproduction signal z is -2, the state S shifts from +1 to -1;, and when the signal z is 0, the state does not shift. Where noise is not included in a reproduction signal, a signal z.sub.k detected herefrom is one of -2, 0, and +2, i.e., a relation of z.sub.k ={-2, 0, +2} exists, and therefore, shift of the state takes place in determined courses. However, a signal Y.sub.k which is obtained by actual detection includes noise n.sub.k. EQU y.sub.k =z.sub.k +n.sub.k ( 3)
If the noise forms a Gaussian distribution, maximum likelihood decoding can be carried out by supposing that a reproduction signal z.sub.k which minimizes an Euclidean distance of (Y.sub.k -z.sub.k).sup.2 has already been supplied.
The maximum value of the sum of each minus Euclidean distance, i.e., {-(Euclidean distance)}, up to a time k at which the state j appears is called a metric (or maximum likelihood) of the state j, which is expressed as L.sub.k.sup.j. When a metric L.sub.k-1.sup.i is obtained in a state i at a time k-1, the metric k.sup.j in a state j at a time k is expressed by the following equation. EQU L.sub.k.sup.j =max{L.sub.k-1.sup.i -(y.sub.k -z.sub.k.sup.ij).sup.2 }(4)
Here, the only one shift which occurs from the state i at time k-1 to the state j at time k and which gives the value L.sub.k.sup.j is stored as a "remaining pass" which generates the highest maximum likelihood, i.e., the highest "probability", and this process is cyclically performed in the Viterbi decoding method. In the case of an NRZI method where the number of states is two, relations of i (or j)=(+1, -1) and z.sub.k.sup.ij ={+2, 0, -2} are obtained (where z.sub.k.sup.ij is used as a reference). These relations are illustrated in form of a trellis chart in FIG. 8.
Thus, a Viterbi decoding method realizes maximum likelihood decoding by which a signal can be decoded depending on a detected signal system which is apart from the signal at the smallest distance and which therefore has the highest probability, so that it is possible to achieve decoding of signals which is less influenced by noise. As a result of this, application of this method into devices of low S/N ratios, such as, a digital recording/reproducing apparatus and the like can be very effective.
FIG. 9 is a block diagram showing the structure of a conventional Viterbi decoding apparatus. In this figure, a signal reproduced by a reproduce device is controlled to have a constant amplitude by an automatic gain control device (AGC) 33. Thereafter, the signal is subjected to waveform equalization by an equalizer (EQ) 34 and is then quantized by an analogue/digital converter (ADC) 35. Digital data thus processed is subjected to a calculation to obtain an Euclidean distance (or branch metric) for every shift by means of a branch metric calculation circuit 36.
In the next, an adder comparator selection circuit (ACS) 37 selects the following pass metric (L.sub.k), in accordance with the formula (4) described below by using the Euclidean distance and a pass metric (L.sub.k-1) of a pass metric memory 38, thereby updating the pass metric and simultaneously recording remaining pass data thus selected, into remaining pass memory means (or pass memory) 39. This pass memory can memory a state shift for a predetermined bit length, and determines a shift state by referring back to past data when a state shift occurs, so that a decoded signal can be outputted. Therefore, an outputted decoded signal may be delayed for a bit length equivalent to the bit length of data which can be stored in the pass memory.
Meanwhile, a pass metric, i.e., the maximum likelihood data used for determining a pass is sequentially changed and stored, and is used again to determine the next pass. In this respect, a Viterbi detection method is different from a conventional detection method in which only the amplitude data at a particular time is used for decoding, and a code series having the highest probability on a time series can be selected by the Viterbi detection method.
However, it will be easy to estimate that, when a pass metric has been greatly changed due to some accidental trouble (which is called a diffusion of the pass metric), such a diffusion will greatly influence on successive decoding to be carried out thereafter. A communication apparatus or a recording/reproducing apparatus of high technologies is frequently used in a situation where a phenomenon like a diffusion of a pass metric often occurs. For example, in a communication system which receives a weak radio wave signal, it is difficult to completely eliminate troubles due to disturbance radio waves which irregularly enter into the system. Or in a digital VTR in which a tape-like recording medium is scanned by a rotation head mounted on a rotation drum, thereby to reproduce a signal, a phenomenon called a drop-out frequently occurs due to damages on the recording medium and a signal is thereby suddenly lost. Therefore, a conventional method requires initialization of a pass metric at a predetermined optimal constant time cycle.
In recent years, a moving image compression method using digital signal processing techniques has been remarkably developed, and attempts to broadcast a large quantity of digital images and to record digital moving images for a long time period have been made by applying the moving image compression method into a digital communication apparatus or a digital recording/reproducing apparatus. Various methods have been proposed as the moving image compression method, and are typically represented by a method which belongs to a so-called block coding variable length compression method. In this method, coding is performed for every small pixel block, and the length of a code is variable in accordance with the data quantity of a small pixel block, so that image compression can be effectively carried out. However, when the compressed block data is transmitted or recorded, and a received signal or a reproduced signal is decoded by a Viterbi decoder, processing for expanding signals after receiving or reproducing the signals is carried out for every compression block, and a diffusion of a metric caused by disturbance signals or a drop-out influences a block next to the compression block which is being subjected to the expansion processing. Therefore, there is a problem that data of blocks which are not concerned with the block influenced by a fault like a diffusion of a metric cannot correctly decoded. In a method in which a metric is initialized at a predetermined time cycle, blocks arbitrarily have various lengths after compression processing, and initialization of a metric therefore may starts in the middle of one compression block, so that sufficient effects for preventing transference of metric diffusion to other blocks cannot be expected. In this respect, it is obvious maximum likelihood decoding cannot be fully effected if the initialization cycle is shortened.
As has been described above, in the Viterbi decoding, a maximum likelihood is calculated from amplitude data obtained at each sampling point, and a data system having the highest likelihood is outputted. However, since only the amplitude data concerning sampling points is used, it would be obvious that outputs are easily influenced by changes in amplitude levels caused by factors other than general noise. This phenomenon is explained in detail in "Digital Video Recording Techniques", page 81, issued by Nikkan Kogyo Shinbun-Sha.
A VTR is a system which brings about changes in reproduction signal level. One of main factors which cause changes in reproduction signal level is a track offset. A track offset occurs since a trace on a tape scanned by a head during recording does not correspond to a trace scanned during reproduction. Therefore, a track offset tends to frequently occur when the same apparatus as used for recording is not used for reproduction or when a tape which has led to changes on the passage of time is reproduced. However, such changes in reproduction amplitude level can be removed with ease by means of an AGC (or automatic gain control circuit) or the like since these changes appear at a relatively long time cycle in accordance with rotation of a head.
Another significant factor which cause changes in reproduction signal level is a clearance space between a tape and a head, i.e., a spacing change. In a VTR, although scanning is performed with a tape and a head being maintained in contact with each other to reproduce a signal, a slight spacing exists between the tape and head because of surface roughness of the tape. The spacing sensitively changes in response to surface conditions or vibrations of a tape, and makes significant influences on the signal amplitude. A drop of the reproduction signal amplitude is called a spacing loss, and logically follows the formula described below. EQU Spacing loss=54.6*d/.lambda.[dB] (5)
(where d denotes a spacing and .lambda. denotes a recording wavelength: a quotation from "Logic of Magnetic Recording", Asakura-Shoten.) It is apparent that a drop in signal amplitude is an exponential function of a spacing and is more greatly effected as the high density recording is carried out with a shorter recording wavelength. It is also apparent from causes of a spacing loss that changes due to a spacing loss occurs at an extremely short cycle. Therefore, with use of a feed back circuit which requires a long convergence time, such as an AGC used for removing changes in amplitude due to track offsets, removal of changes in amplitude occurring at a short cycle is impossible.
Further, changes in spacing result in changes in frequency characteristic. A change in frequency increases equalization errors and causes various errors. In this respect, various developments have conventionally been made to obtain automatic equalizing circuits in which an equalization characteristic is automatically changed such that an equalization error can be converged to be extremely small. These automatic equalizing circuits are naturally used together with a Viterbi detection circuit, and in this case, changes in amplitude can be automatically corrected. However, such an automatic equalizing circuit cannot function to follow changes in frequency characteristic at a cycle shorter than a time required for convergence as far as the circuit is a kind of feed back circuit.
Thus, an AGC circuit or an automatic equalizing circuit does not at all function to respond to immediate changes in reproduction amplitude caused by spacing changes in a VTR, which is a factor rendering a maximum likelihood decoding circuit less functional in a high density digital VTR.